Global auroral pictures from the Defense Meterological Satellite Program (DMSP) satellites are presented to show, for the first time, occurrences of large amplitude undulations on the equatorward boundary of the diffuse aurora in the afternoon-evening sector. The crest-to-trough amplitude of these waveforms ranges from about 40 to 400 km and the wavelength varies from about 200 to 900 km. The undulations are seen in one case to extend over 3000 km along the equatorward boundary. Auroral images from successive DMSP passes suggest that this phenomenon lasts for about 0.5 to 3.5 hours. In each of the four cases observed, the undulations occur during a geomagnetic storm interval near the peak development of the storm time ring current. In all instances, auroral pictures displaying the undulations show simultaneous substorm patterns discrete auroras. In one instance when simultaneous electron (50 eV to 20 keV) measurements from DMSP satellite are available, the electron spectra near the diffuse auroral equatorward boundary resemble power law spectra, and the scale length for the density gradient at the boundary is determined to be about 12 km. Weak electron precipitation is also found equatorward of the diffuse aurora and the associated electron spectra frequently show a secondary population with peak fluxes at 1--5 keV. The observed undulations are interpreted as surface waves propagating on the inner edge of the plasma sheet, and possible plasma instabilities responsible for it are briefly discussed.